1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission which has a plurality of hydraulic engaging elements.
2. Description of Related Art
As this kind of transmission, there has hitherto been known one in which a hydraulic engaging element for a high-speed transmission train is commonly used as a hydraulic engaging element for a reverse transmission train so that the high-speed transmission train and the reverse transmission train can be selectively established by means of a selector gear. As a control apparatus for this kind of transmission, there is known one, as is disclosed in Japanese Published Examined patent application No. 6058/1993, which comprises a servo valve for changing over between forward running and reverse running which is operated to change over the selector gear, and a servo control valve which moves in response to a pressure of the hydraulic oil in a hydraulic engaging element for a low-speed transmission train which is supplied at the beginning of changing over to the forward running range. When the hydraulic oil pressure in the hydraulic engaging element for the low-speed transmission train has increased to a predetermined pressure, the hydraulic oil pressure is supplied to the servo valve via the servo control valve so as to change over the servo valve to a forward running position.
Further, in this control apparatus, in order to alleviate or decrease speed change shocks by buffering a sudden pressure change at the time of supply and discharge of the hydraulic oil to and from respective hydraulic engaging elements, an accumulator is parallelly connected to each of the hydraulic engaging elements. The hydraulic oil pressure corresponding to an engine load such as a throttle opening degree or the like is thus caused to be applied to a back pressure chamber of each of the accumulators.
At an in-gear time (i.e., at the time of gear engagement) to start by changing over from a neutral range to a forward-running range, if the hydraulic engaging element for the low-speed transmission train is suddenly engaged, the driving torque suddenly rises and, by its reaction force, a driving unit made up of an engine and a transmission suddenly displaces in posture through an elastic deformation of engine mounting elements, thereby giving rise to in-gear shocks (i.e., shocks at the time of gear engagement). In this case, the in-gear shocks cannot sufficiently be alleviated or decreased even if the back pressure in the accumulator is controlled depending on the engine load.
In view of the above-described points, the present invention has an object of providing a control apparatus for a hydraulically operated vehicular transmission in which the in-gear shocks can be alleviated by adequately controlling the back pressure in the accumulator.